The present invention relates to the application of polymer coating materials to glass optical fibers, and particularly relates to a new method and new apparatus for applying such coatings to such fibers.
As is well known, polymer and other coatings are customarily applied to glass optical fibers used for optical communications applications in order to protect the fibers from mechanical abrasion which can significantly weaken them. Glass fibers are quite strong as first drawn from a glass melt or preform, but can be drastically weakened by even slight casual contact with solid surfaces, due to the development of tiny flaws on the surfaces of the glass upon such contact. Hence protective coatings are conventionally applied to glass optical fibers immediately after they are drawn, and in any case before they are collected on a winding reel or otherwise brought into contact with other solid materials or each other.
There are at present several commercial processes for the application of polymer coatings to glass optical fibers. Among these are processes involving the application to the fiber of a UV-curable acrylate polymer which can be cured at very high speeds by irradiating the acrylate-coated fiber with intense ultraviolet radiation. Disadvantages of these processes include the relatively high cost of presently available UV-cured acrylate formulations and somewhat less than optimum resistance to thermal deterioration, particularly in certain high-temperature use environments.
More durable coating systems also in commercial use for the production of coated optical fibers comprise the RTV silicone polymers. These materials typically exhibit greater resistance to thermal degradation than UV-curable acrylate polymers, but are generally cured by heating rather than by irradiation. For this reason silicone coatings can not easily be applied at the high fiber drawing rates desired for large-scale commercial optical fiber production.
Another disadvantage associated with many of the commercial coating systems, particularly including two-part or chemically cured systems such as the silicone systems, is the relatively short pot life of the coating mixture. The mixed components of these systems are subject to polymerization even at ambient temperatures, and begin to show gelation accompanied by an increase in viscosity soon after mixing. Thus in conventional commercial practice, after thorough mixing and degasification of the mixture to remove air bubbles, the mixture is cooled during intermediate storage and subsequent application to the fiber.
Even with cooling, large drifts in viscosity tending to produce drifts in the diameter of the coated fiber continue to occur, in essence limiting the length of the manufacturing run time. For example, one commercially used silicone elastomer coating system doubles in viscosity at 25.degree. C. within approximately 2 hours. Heating the resin to reduce the viscosity is not effective since heat accelerates the curing reaction leading to gelation. And ultimately, any material in the coating system which is not applied to the fiber during the draw run must be discarded.
The combined requirements of long pot life and rapid curing characteristics after application to the fiber have largely limited the choices of polymer coating materials which can be applied to optical fibers. Hence while alternative processes such as extrusion have been used to apply stable thermoplastics to optical fibers, such processes have typically been limited to the application of jacketing or overcoating polymers to precoated fibers. Two-package plastic polymer systems other than the silicones have not been commercially used to any large extent because of the difficulty of meeting these requirements.
It is an object of the present invention to provide a method which can be used to coat optical fibers with a variety of polymer systems and which is not limited by the pot life of the selected resin system.
It is further object of the invention to provide a method for coating optical fibers which can employ two-package polymer systems exhibiting very rapid curing characteristics.
It is a further object of the invention to provide apparatus for the manufacture of coated optical fibers which permits the use, for such manufacture, of coating systems exhibiting improved properties such as rapid curing and lower cost.
It is a further object of the invention to provide a method and apparatus for coating optical fibers which reduce the amount of waste material attributable to short pot life in the coating mixture to be applied.
Other objects and advantages of the invention will become apparent from the following description thereof.